The present invention relates to an MR imaging method, a phase error measuring method, and an MRI (Magnetic Resonance Imaging) system, and more specifically to an MR imaging method capable of reducing degradation in image quality due to residual magnetization, a phase error measuring method for measuring a phase error developed due to the residual magnetization, and an MRI system which executes these methods.
The following prior arts have been disclosed in Japanese Patent Application Laid-Open No. H10-75940.
(1) A phase error measuring method of executing a pre-scan sequence for transmitting an excitation pulse, transmitting an inversion pulse, applying a phase encoding pulse to a phase axis, applying a read pulse to a read axis, applying a rewind pulse to the phase axis, subsequently transmitting an inversion pulse, applying a dephaser pulse to the phase axis, and collecting data from echoes while applying a read pulse to the phase axis, and thereby measuring a phase error of each subsequent echo, which is developed due to the influence of eddy currents and residual magnetization caused by a phase encoding pulse, etc., based on phase data obtained by converting the collected data into one-dimensional Fourier form, and
(2) An MR imaging method of, in a pulse sequence of a high-speed spin echo process for transmitting an inversion pulse after the transmission of an excitation pulse, applying a phase encoding pulse to a phase axis, collecting data from echoes while applying a read pulse to a read axis, repeating the application of a rewind pulse to the phase axis plural times while the phase encoding pulse is being changed, thereby collecting data from a plurality of echoes by one excitation, building a compensating pulse for compensating for the amount of the phase error measured by the phase error measuring method described in the above (1) in its corresponding phase encoding pulse, adding the same to one or both immediately before or after the phase encoding pulse, incorporating the same in its corresponding rewind pulse, or adding the same to one or both immediately before or after the rewind pulse.
Each of the prior arts is based on the precondition that the amount of the phase error measured by the phase error measuring method of the above (1) and the amount of the phase error developed when no compensating pulse is added in the pulse sequence of the high-speed spin echo process described in the above (2), are equal to each other.
However, a problem arises in that since the residual magnetization at the commencement of the measurement of the phase error and the residual magnetization at the commencement of the pulse sequence of the high-speed spin echo process are not always equal to each other, the precondition employed in the prior art is not established, and the phase error occurs due to the residual magnetization, thus causing degradation in image quality.
An object of the present invention is to provide an MR imaging method capable of reducing degradation in image quality due to residual magnetization, a phase error measuring method for measuring a phase error developed due to the residual magnetization, and an MRI system for executing these methods.
According to a first aspect, the present invention provides an MR imaging method using a high-speed spin echo process, comprising the steps of transmitting an inversion pulse after the transmission of an excitation pulse, applying a phase encoding pulse to a phase axis, collecting data from echoes while applying a read pulse to a read axis, repeating the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse, collecting data from a plurality of echoes by one excitation, applying a pre-pulse to an arbitrary gradient axis before an excitation pulse, and applying a correction pulse for correcting a phase error caused by the pre-pulse to the phase axis before or behind the initial inversion pulse or adding the amount of correction for correcting a phase error caused by the pre-pulse to the initial inversion pulse.
In the MR imaging method according to the first aspect, the pre-pulse is applied to the arbitrary gradient axis before the excitation pulse. Owing to the pre-pulse, residual magnetization at the commencement of a pulse sequence of the high-speed spin echo process can be controlled. Further, the correction pulse for correcting the phase error developed due to the residual magnetization is applied to the phase axis before or behind the initial inversion pulse, or the amount of correction for correcting the phase error caused by the pre-pulse is added to the initial inversion pulse. Thus, the influence of the residual magnetization at the commencement of the pulse sequence of the high-speed spin echo process can be restrained and degradation in image quality due to the residual magnetization can be reduced.
According to a second aspect, the present invention provides an MR imaging method wherein in the MR imaging method according to the first aspect, the amount of correction for correcting the amount of a phase error caused by a phase encoding pulse is added to its corresponding rewind pulse or phase encoding pulse lying immediately after the rewind pulse or a correction pulse for correcting the amount of a phase error caused by a phase encoding pulse is added before or behind the corresponding rewind pulse.
In the MR imaging method according to the second aspect, residual magnetization at the commencement of a pulse sequence of the high-speed spin echo process can be restrained and the phase error caused by the phase encoding pulse can be corrected, thus making it possible to reduce degradation in image quality, which is developed due to it.
According to a third aspect, the present invention provides an MR imaging method using a high-speed spin echo process, for transmitting an inversion pulse after the transmission of an excitation pulse, applying a phase encoding pulse to a phase axis, collecting data from echoes while applying a read pulse to a read axis, repeating the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse, thereby collecting data from a plurality of echoes by one excitation, applying a pre-pulse to an arbitrary gradient axis before an excitation pulse, and adding the amount of correction for correcting a phase error caused by the pre-pulse to the initial phase encoding pulse or the initial rewind pulse.
In the MR imaging method according to the third aspect, the pre-pulse is applied to the arbitrary gradient axis before the excitation pulse. Owing to the pre-pulse, residual magnetization at the commencement of a pulse sequence of the high-speed spin echo process can be controlled. Further, the amount of correction for correcting the phase error developed due to the residual magnetization is added to the initial phase encoding pulse or the initial rewind pulse. Thus, the influence of the residual magnetization at the commencement of the pulse sequence of the high-speed spin echo process can be restrained and degradation in image quality due to the residual magnetization can be reduced.
According to a fourth aspect, the present invention provides an MR imaging method wherein in the MR imaging method according to the third aspect, the amount of correction for correcting the amount of a phase error caused by a phase encoding pulse is added to the corresponding rewind pulse or phase encoding pulse lying immediately after the rewind pulse or a correction pulse for correcting the amount of a phase error caused by a phase encoding pulse is added to the front or back of the corresponding rewind pulse.
In the MR imaging method according to the fourth aspect, the influence of residual magnetization at the commencement of a pulse sequence of the high-speed spin echo process can be restrained, the phase error caused by the phase encoding pulse can be corrected, and degradation in image quality, which is developed due to it, can be reduced.
According to a fifth aspect, the present invention provides a phase error measuring method comprising the following steps of:
(1) applying a pre-pulse to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, subsequently transmitting a second inversion pulse, applying a dephaser pulse to a phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data;
(2) applying a pre-pulse to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, applying a phase encoding pulse to a phase axis, applying a rewind pulse to the phase axis, subsequently transmitting a second inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data.
(3) applying a pre-pulse to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, applying a phase encoding pulse opposite in polarity to the above (2) to a phase axis, applying a rewind pulse to the phase axis, subsequently transmitting a second inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data;
(4) applying a pre-pulse opposite in polarity to the above (1) to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, subsequently transmitting a second inversion pulse, applying a dephaser pulse to a phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data; and
(5) determining the amount of a phase error caused by the pre-pulse, based on the amount of the phase error obtained from each of the above (1) to (4).
The phase error obtained from the collected data includes three factors of a component xcex94"PHgr"0 (principally caused by residual magnetization at the commencement of a pulse sequence) activated from the commencement of the pulse sequence to the initial inversion pulse, a component xcex94"PHgr"z (principally caused by the residual magnetization at the commencement of the pulse sequence) from the initial inversion pulse to the next inversion pulse or xcex94"PHgr"n (principally caused by a phase encoding pulse), and a component xcex1 caused by a read pulse.
In the phase error measuring method according to the fifth aspect, the amount of a phase error "PHgr"128xe2x88x92 including three factors of xcex94"PHgr"0, xe2x88x92xcex94"PHgr"z and xcex1 is determined from the above (1). The amount of a phase error "PHgr"1 + including three factors of xe2x88x92xcex94"PHgr"0, xe2x88x92xcex94"PHgr"n and xcex1 is determined from the above (2). The amount of a phase error "PHgr"256+ including three factors of xe2x88x92xcex94"PHgr"0, xcex94"PHgr"n and xcex1 is determined from the above (3). Further, the amount of a phase error "PHgr"128+ including three factors of xe2x88x92xcex94"PHgr"0, xcex94"PHgr"z and xcex1 is determined from the above (4). Namely, the following equations are obtained.
"PHgr"128xe2x88x92=xcex94"PHgr"0xe2x88x92xcex94"PHgr"z+xcex1
"PHgr"n+=xe2x88x92xcex94"PHgr"0xe2x88x92xcex94"PHgr"(n)+xcex1
"PHgr"(256xe2x88x92n+1)+=xe2x88x92xcex94"PHgr"0+xcex94"PHgr"(n)+xcex1
"PHgr"128+=xe2x88x92xcex94"PHgr"0+xcex94"PHgr"z+xcex1
If they are solved from the above (5), then the amount of the phase error xcex94"PHgr"0 developed due to the residual magnetization at the commencement of the pulse sequence can be obtained from the following equation.
xcex94"PHgr"0=("PHgr"128xe2x88x92+"PHgr"128+)/2xe2x88x92("PHgr"n++"PHgr"(256xe2x88x92n+)+)/2
According to a sixth aspect, the present invention provides a phase error measuring method wherein in the phase error measuring method according to the fifth aspect, the amount of a phase error caused by the phase encoding pulse is determined based on the amount of the phase error obtained from each of the above (2) and (3).
In the phase error measuring method according to the sixth aspect, the amount of a phase error xcex94"PHgr"(n) caused by a phase encoding pulse can be obtained from "PHgr"n+ determined from the above (2) and "PHgr"(256xe2x88x92n+1)+ determined from the above (3) as follows:
xcex94"PHgr"(n)=("PHgr"(256xe2x88x92n+1)+xe2x88x92"PHgr"n+)/2
According to a seventh aspect, the present invention provides an MRI system, which comprises RF pulse transmitting means, gradient pulse applying means and NMR signal receiving means, which executes MR imaging by a high-speed spin echo process for controlling the respective means to thereby transmit an inversion pulse after the transmission of an excitation pulse, apply a phase encoding pulse to a phase axis, collect data from echoes while a read pulse is being applied to a read axis, repeat the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse and collect data from a plurality of echoes by one excitation, and which includes pre-pulse applying means for applying a pre-pulse to an arbitrary gradient axis before the excitation pulse, and pre-pulse correcting means for applying a correction pulse for correcting a phase error caused by the pre-pulse to the phase axis before or behind the initial inversion pulse or adding the amount of correction for correcting the phase error caused by the pre-pulse to the initial inversion pulse.
The MRI system according to the seventh aspect can suitably execute the MR imaging method according to the first aspect.
According to an eighth aspect, the present invention provides an MRI system, which in the MRI system according to the seventh aspect, includes phase encoding pulse correcting means for adding the amount of correction for correcting the amount of a phase error caused by a phase encoding pulse to the corresponding rewind pulse or phase encoding pulse lying immediately after the rewind pulse or adding a correction pulse for correcting the amount of a phase error caused by a phase encoding pulse to the front or back of the corresponding rewind pulse.
The MRI system according to the eighth aspect can suitably execute the MR imaging method according to the second aspect.
According to a ninth aspect, the present invention provides an MRI system, which comprises RF pulse transmitting means, gradient pulse applying means, and NMR signal receiving means, which executes MR imaging by a high-speed spin echo process for controlling the respective means to thereby transmit an inversion pulse after the transmission of an excitation pulse, apply a phase encoding pulse to a phase axis, collect data from echoes while a read pulse is being applied to a read axis, repeat the application of a rewind pulse to the phase axis plural times while changing the phase encoding pulse and collect data from a plurality of echoes by one excitation, and which includes pre-pulse applying means for applying a pre-pulse to an arbitrary gradient axis before the excitation pulse, and pre-pulse correcting means for adding the amount of correction for correcting a phase error caused by the pre-pulse to the initial phase encoding pulse or the initial rewind pulse.
The MRI system according to the ninth aspect can suitably execute the MR imaging method according to the third aspect.
According to a tenth aspect, the present invention provides an MRI system, which in the MRI system according to the ninth aspect, includes phase encoding pulse correcting means for adding the amount of correction for correcting the amount of a phase error caused by a phase encoding pulse to the corresponding rewind pulse or phase encoding pulse lying immediately after the rewind pulse or adding a correction pulse for correcting the amount of a phase error caused by a phase encoding pulse to the front or back of the corresponding rewind pulse.
The MRI system according to the ninth aspect can suitably execute the MR imaging method according to the third aspect.
According to an eleventh aspect, the present invention provides an MRI system, which in the MRI system according to each of the seventh through tenth aspects, includes phase error measuring means, which performs the steps of:
(1) applying a pre-pulse to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, subsequently transmitting a second inversion pulse, applying a dephaser pulse to a phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data;
(2) applying a pre-pulse to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, applying a phase encoding pulse to a phase axis, applying a rewind pulse to the phase axis, subsequently transmitting a second inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data.
(3) applying a pre-pulse to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, applying a phase encoding pulse opposite in polarity to the above (2) to a phase axis, applying a rewind pulse to the phase axis, subsequently transmitting a second inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data;
(4) applying a pre-pulse opposite in polarity to the above (1) to an arbitrary gradient axis, transmitting an excitation pulse, transmitting a first inversion pulse, subsequently transmitting a second inversion pulse, applying a dephaser pulse to a phase axis, collecting data from echoes while applying a read pulse to the phase axis, applying a rephaser pulse, subsequently transmitting a third inversion pulse, applying a dephaser pulse to the phase axis, collecting data from echoes while applying a read pulse to the phase axis, and determining the amount of a phase error, based on the collected data; and
(5) determining the amount of a phase error caused by the pre-pulse, based on the amount of the phase error obtained from each of the above (1) to (4).
The MRI system according to the eleventh aspect can suitably execute the phase error measuring method according to the fifth aspect.
According to a twelfth aspect, the present invention provides an MRI system wherein in the MRI system according to the eleventh aspect, the phase error measuring means determines the amount of a phase error caused by the phase encoding pulse, based on the amount of the phase error obtained from each of the above (2) and (3).
The MRI system according to the twelfth aspect can suitably execute the phase error measuring method according to the sixth aspect.
According to an MR imaging method of the present invention, the influence of residual magnetization at the commencement of a pulse sequence of a high-speed spin echo process can be retrained and hence degradation in image quality can be reduced.
According to a phase error measuring method of the present invention, it is possible to suitably measure a phase error caused by the influence of residual magnetization at the commencement of a pulse sequence of a high-speed spin echo process.
Further, according to an MRI system of the present invention, the above-described method can suitably be executed.